Flip-chip mounting and the like are conventionally employed when electronic components and the like are mounted on a circuit substrate. During the flip-chip mounting, a bump is formed on the circuit substrate. In a method for forming the bump, for example, metal plating is performed, and electroplating is particularly performed.
For example, Patent Literature 1 discloses a method for forming a metal bump, in which electroplating is performed. In particular, in order to form the metal bump by electroplating, a resist pattern having openings is firstly formed on the circuit substrate. The metal bump comprising metal plating film(s) is subsequently formed on the circuit substrate inside the openings by electroplating. By using such a method, a height of a metal bump can be controlled depending on a thickness of a resist. For example, a metal bump having a high aspect ratio can be formed by increasing a thickness of a resist.
As a material for a metal bump, used are various metals such as lead (Pb), bismuth (Bi), zinc (Zn), copper (Cu), and silver (Ag), as well as tin (Sn). In recent years, an alloy of such metal components has been, in particular, frequently used as a material for a metal bump. For example, prepared is a plating solution containing two or more metals mentioned above, and then an alloy plating film is formed from the plating solution by electroplating. As a result, an alloy bump can be formed.
However, a problem has emerged due to recent miniaturization of electronic circuits. That is, during forming of an alloy bump having a high aspect ratio by increasing a thickness of a resist, one metal may be hard to deposit on the bottom of openings of the resist, on which the bump is to be formed, while the other metal may deposit in large quantity on the bottom. As a result, plating composition becomes to be unstable. In order to solve such problem, for example, a method for stabilizing metal composition of a Sn—Ag alloy bump is advanced in Patent Literature 2. In the method in Patent Literature 2, during forming of an alloy plating film from a Sn—Ag alloy plating solution by electroplating, acid concentration of the plating solution is monitored, and Ag concentration in the plating solution is adjusted based on the acid concentration. Thus the amount of deposited Ag can be maintained constant, and the metal composition of the alloy bump can be maintained constant.